How does a Cs-fountain clock work?

A clock is a timekeeping device. In the past, people used the shadow of a building, the position of some fixed stars, the sun, the moon and some other heavenly bodies to keep track of time. As the human civilization progressed, they developed some devices like a water clock and a sand clock to know time. In the early seventeenth century, Galileo discovered that a swinging pendulum can be used as a time keeping device. Inspired by this discovery, Christian Huygens invented a pendulum clock in the mid seventeenth century. These clocks can still be seen used in several places. In the mid twentieth century, it was discovered that atoms can be used to keep time. The 1955 Cesium Atomic Clock with a Cesium beam tube developed at the National Physical Laboratory, UK, kept time to a second in 300 years. To increase the accuracy of the clocks, the interrogation time had to be increased. This could be done by decreasing the speed of the atomic beam or by increasing the length of the the tube. But the problem with the increasing the length of the tube was that the atoms would form a sag in travelling through the tube due to gravitational potential. A new idea was developed where the tube could be rotated so that it would be in a vertical position and the atomic beam could be projected vertically upward. This new configuration along with the development of the laser cooling techniques developed in early nineties made the modern, highly accurate Cesium fountain clocks possible. The modern Cesium clocks developed at NIST, Boulder, Colorado, USA would neither gain nor lose a second in more than 60 million years.

Note: This posting is in progress.

Physics Journals

The following links contain a number of physics and astrophysics journals published all around the world:

(1) http://www.slac.stanford.edu/spires/coden/list.shtml

(2) http://www-library.desy.de/eljnl.html#physics

(3) http://www.ifpan.edu.pl/journal.html

(4) http://journals.aip.org/

Bosons or Fermions?

A class of particles which have an integer spin are called bosons. Example - photon, etc. Any number of bosons can go to the same quantum state. Thus they are friendly to each other ! They obey Bose-Einstein statistics. The wave function associated with bosons is symmetric.

A class of particles which have a half- integer spin are called fermions. Example - proton, neutron, electron, etc. Unlike bosons, only two fermions (at maximum) can go to the same quantum state, as dictated by the Pauli Exclusion Principle. They obey Fermi-Dirac statistics. The wave function associated with fermions is anti-symmetric.

An atom can also be classified as a composite boson or a composite fermion.  To find whether an atom is a composite boson or a composite fermion, you need to look at the net spin of the atom due to its constituent particles that make it. For example, consider the simplest of the atoms - Hydrogen. Hydrogen has a proton and an electron. A proton is a half-integer particle and so is an electron. Therefore, the net spin of a normal hydrogen atom is one, which is an integer. Therefore, hydrogen is a composite boson. If we consider a helium-4 atom, there are two protons, two neutrons and two electrons. Each of these particles has a half integer spin. Therefore, the net spin of a normal helium atom is an integer. Hence, helium is a composite boson. What's about lithium-7 ? A lithium-7 atom has three protons, four neutrons and three electrons. Therefore, the net spin of a lithium-7 atom is an integer and hence it is a composite boson. On the other hand, by the same way of reasoning, lithium-6 is a composite fermion.

In general,  an atom can be classified as a composite boson or a composite fermion on the basis of  the total number of constituent particles contained in it. If the total number of constituent particles  is even it is a composite boson where as if the total number of constituent particles is odd, it is a composite fermion !

Gordon Research Conferences - 2009 (Atomic Physics)

The gordon research conferences take place on a number of frontiers in research areas every years and the conference in the same area is organized every two years. They were started by Professor Neil E. Gordon in 1930s. Therefore, these confernces carry a long hostory with them. The GRC-2009 in Atomic Physics was organised from June 28-July 03 at Tilton Shcool, Tilton, New Hampshire.

More than 150 participants incuding twenty plus speakers were present in the conference. The speakers were the top researchers in the field from around the world. About a 100 posters were presented on current researches in two sessions. Most of the talks and posters were from the experimentalists on the subject but there were some atomic physics theorists too to give talks and present posters. The talks were on variety of disciplines of Atomic physics - Bose and Fermi gases, atomic reactions in ultra cold environments, formation of qubits using atomic ions, etc. A day started with breakfast at 7:30 A.M. and ended with a social from 10:00 P.M. - 12:00(midnight) or so. I was so surprised to see that the frontier research scientists work all the time no matter whether they are in a meeting hall or in a dining table or in a social or wherever they are !

There were a lot of indoor and outdoor extra activities too in the free time like hiking, rafting and kayaking. Unfortunately, because of the weather, we could not do outdoor activities. Most of the week was spent on campus, attending the conferences, presenting poster, chatting, working on computers and eating and drinking.

I can not stop myself writing on the quality and quantity of foods in the conference. It was the place where you could eat anything of your choice and any amount you would want. It was really great. The conference staffs in the kitchen and everywhere were so friendly and helpful. We spent a very good time in overall in the GRC-2009 in Atomic Physics at Tilton School !

Second Annual Greater Boston Area Quantum Matter Meeting

The Second Annual Greater Boston Area Quantum Matter Meeting took place in Metcalf Science Center, Physics Department, Boston University, on Saturday, May 9, 2009. The topics of presentation were mainly on quantum systems: strongly correlated systems, atomic and optical systems, and mesoscopics. There were four invited talks, each of about 30 minutes duration and more than thirty contributed talks, each of 4-5 minutes. The schedule is available here. This program was a continuation of the First Annual Greater Boston Area Quantum Matter Meeting, which took place in Jefferson Building, Physics department, on Saturday, May 10, 2008. The link of this meeting can be found in here.

How is the information extracted from cold atom interferometers?

The cold atoms (thermal atoms) or a BEC sitting at the bottom of a magnetic trap is split by a laser standing wave and the wave packets are allowed to evolve in time. At the end of the interferometric cycle, the wave-packets are recombined by a recombining pulse (identical to the splitting pulse). The trap is switched off and the wave packets are allowed to expand. Then the imaging of the wave packets is done by using laser light. One of the techniques is the absorption imaging technique. In this method, a resonant light is shone on the wave packets and the images obtained from this are fitted with some suitable known models. The useful information is then extracted by interpreting the fitting parameters.

Note: This post is in progress !

BEC in a triple well potential

This is a review of a paper by Rab et. al.

Consider a triple well potential with the wells 'L' , 'M' and 'R' for the left, middle and right wells respectively. A BEC sitting in the 'L' well can be transported to the 'R' well so that no atoms are left in the 'M' well. The researchers call this process as macroscopic matter-wave Transport Without Transit (TWT) and this can be done by Stimulated Raman Adiabatic Passage(STIRAP). In STIRAP, is a technique to transfer population between two atomic states - 1 and 3 via an intermediate excited state 2. The atomic population is adiabatically transferred from the state 1 to state 3 by coupling the states 1 to 2 and and 2 to 3 using electromagnetic pulses. The population transfer is achieved via a superposition of the states 1 and 3 with the occupation of the state 2 strongly suppressed. That's why is is called the TWT.

Note: This post is in progress.